[cpp-threads] Asynchronous Function Proposal

[Peter Dimov]

Herb Sutter:
>> - execute immediately (for generic contexts)
>
> Yes, but that is needed (only?) for testing and would be routinely 
> provided as a QoI extension. No harm in providing it though.

This is handy when you want to e.g. limit the nested parallelism when 
splitting recursively.

void f( int level, int n, ... )
{
    async( level < 5 && n >= 10? default_policy: synchronous_policy, f, 
level+1, n / 2, ... );
}

In this case you can just do an 'if', but more complicated examples might 
have a function that determines the policy based on some criteria, 
encapsulating the logic, or take a policy hint and modify it, and so on.

>> - defer until wait() is called (not get)
>
> I'm not opposed to including something like this, but I have seen no 
> experience with it or need for something finer-grained than "execute 
> immediately." Is there?

I'm not sure; this is just a "fixed" variant of Lawrence's deferred policy. 
It might be useful for implementing a form of lazy evaluation. I remember a 
boost::future proposal by Braddock Gaskill including such a capability.